This invention generally relates to treatment of largely aqueous solutions at acid pH values to remove suspended solids. In particular, this invention involves treating the recycle stream of a chrome tanning bath with magnesium silicate to promote removal of suspended solids while minimizing loss of chrome values.
Hides of various animals are treated mechanically and chemically to produce leather. The most common method of tanning is chrome tanning wherein the hides, after a number of preparatory steps, are placed in a bath containing chromium salts such as chromium (III) chloride or chrominum (III) sulfate. In the bath, the hides absorb the chromium (III) ions which render the hide into leather.
The exhausted tanning bath contains significant chrome values along with fats, oils and suspended solids. The chrome values can be partially recovered by a recycling system of several steps. The oil and fat may be removed mechanically by skimming. Then, lime and a flocculating agent added to flocculate the chrome and other suspended solids. The supernatant from this step is discarded. The flocculated solids containing the chrome are acidified to redissolve the chrome value; unfortunately, the other flocculated solids also become redispersed. Then additional chromium salt is added and the bath is ready for use. As the bath is recycled, the suspended solids build up and after a number of these cycles, the spent bath must be discarded with the consequent substantial loss of expensive chrome values.
Numerous flocculating agents of the type used in water treatment have been used in an attempt to remove suspended solids from the recycle stream without removing the chrome value. Unfortunately, these agents do not function well in the acid environment of the chrome tanning bath.
It is an object of this invention to remove suspended solids from tannery chrome recycle streams without substantially reducing the concentration of the chrome value in that stream. It is a further object of this invention to extend the number of recycles of the chrome bath before it must be discarded, thereby saving considerable chrome values and money.
In preparing this application, the fields of tanning, waste water treatment, flocculants and magnesium silicate were considered and searched. No relevant references were found thereby no prior art references were considered when drafting this application.